Typically the ring gear is welded to the convertor, replacement requires exact precision best left to the professionals. It's be cheaper, faster & easier to just outright replace the entire convertor. If power, speed and performance are important to you, might I suggest getting a new convertor with a higher-than-stock stall speed? Your engine will rev a little higher than before, allowing it to develop more torque and accelerate faster. C-bodies are big, heavy cars and can use all the help they can to get rolling from a dead stop. Now this can lead to higher temperatures, so a trans cooler and/or a deeper oil pan (with cooling fins) can help maintain and assure longevity. (I did both, plus a magnetic drain plug too) For street driving, I wouldn't go any higher than 400 over stock or it'll slip a lot at highway cruising speeds and get crappy gas milage.
Have you replaced the starter yet? Was it just wear and tear of being old, or was a bolt missing or loose leading to misalignment of the starter?
